In the automatic testing of integrated circuits (IC) and other electronic devices, special equipment may be used to bring the device to the proper temperature and place the device to be tested in position. The electronic testing itself is provided by a large and elaborate automatic testing system which includes a test head. The test head is often densely packaged with electronic circuits in order to achieve the accurate high speed testing of the sophisticated devices. The testing system with the test head has been required to connect to and dock with a device handler or other equipment for supporting the electronic devices. Test head positioning systems are well known in the field of automatic testing of semiconductor devices.
In such testing systems, the test head is very heavy, on the order of 40 to 500 kilograms. Recently, test equipment has been introduced which use test heads of upwards of 1000 kilograms or more. The reason for this heaviness is that the test head uses precision high frequency control and data signals so that the electronic circuits may be located as close as possible to the device under test.
For purposes of this explanation, a unit under test (UUT) is a device such as a circuit or subassembly undergoing a test. A device under test (DUT) is the present device (IC) undergoing test. The DUT is a particular type of unit under test (UUT). A test station apparatus (TSA) refers collectively to wafer probers, device handlers and manual test stations. A test head is an operating head which is specifically designed for testing.
Recent advances in the semiconductor industry have had the following effects:
1. DUTs have become more complex; the number of transistors per device has grown steadily from a few thousand to a few million.
2. DUTs have become increasingly complex mixtures of digital circuitry, analog circuitry, and mixed signal circuitry.
3. The number of I/O, power supply and signal ground reference pins per device has grown to several hundred from less than one hundred
4. DUT clock rates have gone from 10's of Mhz to at least 1 Ghz.
5. Data, Address and Control signals are accordingly in at least the 100's of Mbps range.
6. Per pin frequency bandwidth requirements are at least in the 10's of Ghz range.
As a result, desired test system design changes are made to accommodate the following considerations:
1. The test head includes more, faster and more complex pin-electronics circuits.
2. Greater power dissipation per pin circuit in the test head can be expected due to the higher switching rates.
3. Water cooling systems with circulating water have been added to the test heads, increasing their weight and necessitating flexible plumbing to be included in the test head cable.
4. More power supply and ground conductors of heavier gauges are in the interconnecting test head cable.
5. Circuitry is relocated from the system cabinet to the test head to reduce the amount of signal wiring and associated delays in the cable.
These factors have driven the size and weight of test heads to increase significantly:
1. The demand that the test head be brought into close proximity to the DUT is increasingly important.
2. The cable from test head to tester cabinet should be kept as short as possible while at the same time it has become thicker and stiffer.
Thus, test heads and their associated cables have become significantly larger and heavier.
The test head positioning system is a mechanical device that allows an operator to bring an automatic test equipment test head weighing hundreds of pounds into proximity with a device handler, wafer prober or other test station apparatus (TSA) and to allow the test head to be docked with the TSA. As semiconductor chips have and continue to become faster and more complex, the size and weight of test heads have continually grown larger. What is more, the size and weight of the cable that connects the test head to the automatic test equipment cabinet have correspondingly increased. These factors have created demand for test head positioning systems with both an increased load carrying capacity and an increased range of motion. Overall it is very challenging to design an apparatus that can freely move loads approaching many hundreds of pounds and that exert moments in the range of 100,000 inch-pounds or more on the supporting structure.
The test head positioning system may be used to position the test head with respect to the device handler or other test station apparatus (TSA). When the test head is accurately in position with respect to the TSA, the test head and the TSA are said to be aligned. When the test head and TSA are aligned, the fragile test head connectors and TSA electrical connectors can be brought together (i.e., docked), enabling the transfer of test signals between the test head and the TSA. Prior to docking, the fragile test head connectors and TSA electrical connectors must be precisely aligned to avoid damaging the fragile electrical connectors.
A test head positioner system may also be referred to as a test head positioner or a test head manipulator. This technology is described, for example, in the inTEST Handbook, inTEST Corporation. This technology is also described, for example, in U.S. Pat. Nos. 6,057,695, 5,900,737, 5,608,334, 5,450,766, 5,030,869, 4,893,074 and 4,715,574 which are all incorporated by reference for their teachings in the field of test head positioner systems.
Test heads, and their respective positioners, are often used in a clean room environment. Clean room environments are often expensive to provide, and so the useable space is available at a premium. A variety of test head positioning systems are currently available for use in clean room environments. Although some of these test head positioning systems have a variety of desirable features, the amount of space which each of these test head positioning systems requires for proper operation may be undesirable. As device testing has developed to handle ever more increasingly complex tasks, test heads have continued to increase in size and weight. As test heads get larger and larger, fully manual, fully balanced methods become ever more difficult to actually implement in hardware, and further improvements are called for.
Ideally, a heavy load such as a heavy test head would be supported in a substantially weightless condition for relevant movements. “Substantially weightless” means that force required to position the heavy load is substantially less than that which would be required to lift the load.
The test head positioning system innovations disclosed in U.S. Patent Application Serial No. PCT/US00/00704 (which is incorporated herein by reference) have proved helpful in many situations. These innovations have provided a fully balanced manipulator that provides six degrees of motion freedom to the test head. The system is counterbalanced and the test head cable is managed so that the test head can be easily moved in all six degrees of freedom with little force. This allows an operator to easily position the test head and allows the test head docking mechanism to readily draw the test head in to precise alignment with the TSA test site. Also independent motion in each degree of freedom is facilitated.
When the test head is in motion, the total mass in motion includes the test head, the counter weights, and all moving portions of the manipulator. The total mass in motion is much greater than just the mass of the test head and is correspondingly very high. Operators can be intimidated and fearful of manually manipulating such a massive object and apparatus, even if it is fully balanced and requires only a small amount of force. Incorporation of powered axes with remote controls has become a desirable feature; however, while docking, the dock actuation mechanism takes over control of the manipulator; and it is then preferred to have the manipulator in a compliant condition to allow the test head to freely move with little force in all axes simultaneously. Many users prefer to leave the manipulator in a free-to-move, balanced condition while testing to absorb vibrations from the test handling automation equipment.
The increased range of motion requirements combined with the increased structural requirements lead to the situation where the manipulator can become too large to be shipped conveniently. In particular the maximum height of a shipping container and pallet with skids is limited to approximately 109 inches (approximately 9 feet) by existing trailer trucks and the largest commercial cargo airplanes (Boeing 747s). Overall vertical motion requirements are presently approaching 48 inches; when combined with the structural requirements for the base and column, and assuming a vertical column of fixed height, this leads to a system that is too tall to be conveniently shipped.
A solution to the height problem is to utilize a column made of two or more segments that can be collapsed, folded or telescoped. Manipulators with telescoping vertical columns have been known for several years. Indeed, until recently, the extra costs of constructing a telescoping vertical column have not been justifiable. Much more recently, U.S. Pat. No. 5,931,048, to Slocum et al, and U.S. Pat. No. 5,949,002, to Alden, disclose a manipulator having a motor driven telescoping column. In '048 a pneumatic mechanism is used to provide vertical axis compliance for docking. The '002 patent observes that such a solution has limitations and seeks to improve compliance by using a load cell as a sensor in an automatic control system feedback loop. This active compliance technique adds to system cost and complexity, particularly when safe operation in the event of failures and unplanned obstructions to motion are considered. A more desirable arrangement would be a motor driven vertical axis, which may or may not be of the telescoping type depending upon system requirements, that includes counterweights in a fashion that allows essentially weightless counterbalanced motion for docking and testing operations.
Present manipulators which use counter weights typically are configured so that the total weight of the counter weights is approximately equal to the load. The load consists of the test head, its movable support apparatus, and a portion of the cable. Thus the counter weights can be more massive than the test head itself. As test heads become heavier this leads to certain undesirable situations as follows:                The volume required to house the weights within the manipulator becomes unmanageable.        The total amount of weights that must be handled in the installation of a new test head as well as the storage of unused weights becomes likewise unmanageable.        System inertia and certain restraining frictional forces also vary as approximately two times the test head weight plus the weight of movable manipulator components.        Loading effects on column, platform, cable pulleys, and other subassemblies grow at twice the growth rate of test head weight. These loading effects can also have an adverse effect on overall system friction and, consequently, the force required to position and dock the test head.        The movable plates and other apparatus of a movable base unit have appreciable mass which grows with test head weight and which contributes significantly to total system inertia.        
Positioning heavy objects is generally very difficult. Manipulation of heavy objects is typically performed with power assist, by servo mechanisms and by load balancing. Each of these techniques has disadvantages. In particular, if an operator of equipment manipulating a heavy object cannot completely control relevant movement, the result could be serious bodily injury or equipment damage.
In some cases, the load may have a slight bias, so that only a small fraction of the total weight of the load would be reflected in the force required to adjust a position of the load. Thus if a 300 kg load were to be positioned, it would be very difficult to move the load without mechanical or power assistance. If the force required to move the 300 kg load were 5 kg, it would be much easier to manipulate the load. In addition, there is substantially reduced danger to the machine operator who may get caught between the load and another object. Since the 5 kg is insignificant as compared to the 300 kg, the load is considered to be suspended in a “substantially weightless” condition.
A positioner, able to move along a support structure, carries the test head to the desired location at which the test head is positioned to connect to and dock with the TSA. The test head is attached to the positioners so that the test head can achieve up to six degrees of motion freedom: X, Y, Z, θX, θY, θZ.
To be consistent with descriptions of test head positioner systems in the prior art, the coordinate system 100 illustrated in FIGS. 2 and 3 is used in which:                Y=vertical, up-down axis 102        X=horizontal, side-to-side or left-right axis 104        Z=horizontal, in-out axis 106.        
When viewed from the front of the manipulator, this forms the Cartesian coordinate system.
Rotations about the axes are designated as follows:                θY (theta Y)=rotation about the Y axis (hereinafter referred to as “swing” or “yaw”) 108        θZ (theta Z)=rotation about in-out axis (hereafter referred to as “roll” or “twist”) 110        θX (theta X)=rotation about X axis (hereinafter referred to as “tumble” or “pitch”) 112        
Referring specifically to FIG. 3, the test head 305 used in a preferred embodiment of the present invention includes a test interface 310 on one surface. The test interface 310 typically holds a test interface board 315, which in addition to other functions, provides connection points to the test apparatus in the prober, handler, manual test station, or other test station apparatus (TSA). As shown in FIG. 3, the test head 305 has a length 312, a thickness 313, and a width 314. The surface having the test interface 310 is hereinafter called the “interface surface” (IS) 310. The IS defines both a plane and a direction; the direction is described by a vector normal to the IS plane and pointing away from the test head. FIG. 3 shows the IS 310 in the horizontal plane with upwards direction. In use, the orientation of the test head will change as it is docked with different types of equipment. Accordingly, both the plane and the direction of the IS 310 will also change.
In use, the test head may be brought upwards into contact with the DUT as in the TSA. In this case, the interface surface 310 is facing up (IS UP). Another alternative is that the test head may be brought downward into contact with the device as in a prober. In this case, the interface surface 310 is facing down (IS DOWN). Thirdly, both the DUT and the IS 310 may be oriented in a vertical plane (IS vertical). Fourthly, both the DUT and the IS 310 may be oriented at any angle between the IS up and IS down. As the test head is moved from one apparatus to another, it is clear that the test head must be rotated among IS UP, IS DOWN, and IS Vertical positions. Depending upon the manipulator, this may be effected by rotation in either the tumble (pitch) or roll (twist) coordinates.
The test head is connected to the test cabinet by a large, thick bundle of cables (not shown). The cables, size, weight and resistance to flexing and twisting all interfere with and constrain test head motion. Test head motion is desirably performed in a manner to protect the cable from several factors as described, for example in U.S. Pat. No. 5,608,334.
Many test heads, particularly large ones, are attached to the manipulator using a cable-pivot apparatus. In the cable-pivot mode, the cable is brought from the test head at the center of rotation and parallel with the axis of rotation (normally the roll or twist axis). This is described, for example, in U.S. Pat. Nos. 5,030,869, 5,450,766 and 5,608,334. A mechanism to support the cable is described, for example, in U.S. Pat. No. 4,893,074.
Several manipulators provide two motions or more in their bases. A manipulator manufactured by Teradyne and described in U.S. Pat. Nos. 5,931,048 and 5,949,002, referenced above, use such a motion. The Teradyne device provides swing rotation and side-to-side motion in its base with the swing mechanism at the bottom; in-out motion in this manipulator is provided in the arm and cradle assemblies. A further manipulator manufactured by Reid-Ashman Mfg., Inc., provides side-to-side and in-out motion in the base with swing provided by rotation of the arm about a shaft attached to the column. A manipulator manufactured by Schlumberger provides side-to-side motion and swing rotation motion in its base with the swing mechanism on top of the side-to-side mechanism. In-out and roll motions are provided in the arm assembly. This manipulator is unique in that the cable passes from the cabinet, through the center of the column above the center of rotation, to terminate at the test head. Also unique to this manipulator is a spring mechanism that serves as a counter balance which was first described in U.S. Pat. Nos. 4,943,020 and 4,973,015, both assigned to Schlumberger. Finally, a further manipulator, also manufactured by Reid-Ashman Mfg., Inc., provides all three motions in its base with the swing rotation mechanism at the bottom.
Drive mechanisms such as motors and other actuators can be classified as “compliant” and “non-compliant” according to the tendency of a force on the driven object to move the drive mechanism. For the purposes of this specification, if a force on the driven object can easily move the drive mechanism, then the drive mechanism is considered to be “compliant.” If a force on the driven object is normally unable to move the drive mechanism, the drive mechanism is considered to be “non-compliant.”
In mundane terms, if a drive motor can be readily driven forward or backward by forcing on the driven object at its output, the drive motor is “compliant.” If forcing on the driven object will break the drive mechanism, or if an inordinate amount of force is required to force the drive motor by the driven object, the drive motor is “non-compliant.” This is often the case with geared drive motors. Usually non-compliant drive mechanisms have high drive reductions, so that, for example 1000 turns of a motor will result in 1 turn of a driven gear. The non-compliant nature of drive mechanisms is usually the result of gear reduction, in which the small amounts of friction or latent magnetic force will prevent back driving of the mechanism. It is also possible to make a drive non-compliant with valves or brakes.
In order to provide all of the advantages of a fully balanced manipulator in all six degrees of motion freedom, it is desired to allow fully balanced, substantially weightless, displacement-independent vertical motion for test head docking in a test head manipulator. It is also desired that the test head manipulator have its main vertical motion driven via a screw or other non-compliant mechanism. Furthermore, to reduce loading on the vertical column and on its base, it is desired that the total weight of the counter weights is to be significantly less than the combined weight of the test head, cable load, and support apparatus. It is also desired to have a technique that may be used with manipulators where the main vertical motion is provided by a telescoping column as well as with fixed-column manipulators, where the test head is moved along a column of fixed height. Because precision test head docking requires simultaneous motion in all six degrees of freedom, it is desired that motion freedom be maintained in the other five degrees of freedom simultaneously with the vertical motion.
Regardless of the particular means to control motion and counterbalancing of the motion, it is desirable that accidental collisions be avoided and if an accidental collision does occur, the powered mechanism be able to react in order to minimize the effects of the collision. Sensing and reacting to accidental collisions is a particular problem when multiple drive mechanisms are used to achieve movement in one direction.
Further objectives are to provide safety features for both the operator and test head. From a safety standpoint, it is desired to provide adequate structural support for the test head so that it cannot fall to the ground or through a distance of more than approximately two inches in the event that the counterbalance mechanism fails or becomes unbalanced. A second safety objective is to have the ability to detect undue resistance to vertical motion from any causes including collisions with another piece of apparatus or with a human, interference by improper human interaction, collision or interference with the docking mechanism, etc.
Test head positioners, like other heavy machinery, are built with safety considerations inherent in their design. Larger, heavier test heads lead to greater chances of accidental collisions due to the unplanned physical displacement of other test floor apparatus into the planned test head motion envelope, humans unexpectedly getting into the path of motion, operator errors, control system malfunctions, and the like. With more-massive, more-expensive test heads, the damage to property, equipment and humans that can be caused by an accident can be significant and even lethal. In the case of manually powered motion, the human operator can typically feel an obstruction and can accordingly stop or reverse motion and avoid damage. With a powered manipulator axis that is remotely controlled by either an operator or by an automatic control system, special means must be employed to detect an obstruction or collision and to avoid causing significant damage. In U.S. Patent Application Serial No. PCT/US00/00704, it is taught that if the controlled axis is balanced so that only a minimal amount of force is required to cause motion, then current limited dc motors can be effectively used. Because an obstruction, even a soft one, will cause the force required for motion to increase, the motor will stall and the force transferred to the obstruction will be limited to a safe amount.
If the main vertical motion is to be unbalanced, the vertical drive must lift the entire weight of the test head and its supporting apparatus. The drive motors would not necessarily notice the additional force imposed by an obstruction. Accordingly, if the manipulator is to have a driven main vertical motion that is not counter balanced combined with a secondary, or vernier, vertical motion for docking, it would be highly desirable to include a means for detecting obstructions and collisions while the main vertical drive is in operation. As used in this description, “vernier” refers to a small auxiliary device used with a main device to obtain fine adjustment.